1. Field of the Invention
The present invention relates to a power semiconductor device which is sealed with a mold resin and a method of manufacturing the power semiconductor device. Particularly, the power semiconductor device of the present invention comprises an insulating sheet which is a complex of an insulating resin layer and a metal layer.
2. Description of the Background Art
The first prior art of the mold resin-sealed power semiconductor device is a device disclosed in Japanese Patent Application Laid-Open No. 2000-138343 (Patent Document 1). In this prior-art device, a power chip is fixed onto a first frame with solder and a surface electrode of the power chip and an electrode provided on a second frame are interconnected with a wire. The first and second frames are entirely covered with a mold resin.
The second prior art of the mold resin-sealed power semiconductor device is a device disclosed in Japanese Patent Application Laid-Open No. 2001-156253 (Patent Document 2). In this prior-art device, a power chip is fixed on an upper surface of a frame and a lower surface thereof is in direct contact with or fixed onto an insulating substrate 201 with high thermal conductivity which is made of Al2O3, AlN, BeO or the like, and then the frame, the power chip and the insulating substrate are sealed with a mold resin.
The third prior art of the mold resin-sealed power semiconductor device is a device disclosed in Japanese Patent Application Laid-Open No. 2002-76204 (Patent Document 3). In this prior-art device, an insulating resin layer formed of a sheet-like thermosetting resin is fixed on an upper surface of a metal radiator plate whose lower surface is exposed outside and after that, an unhardened bonding layer is formed on the insulating resin layer and a frame on which a power semiconductor chip is mounted is further bonded to the bonding layer, and then the whole is sealed with a mold resin.
The fourth prior art of the mold resin-sealed power semiconductor device is a device disclosed in Japanese Patent Application Laid-Open No. 10-125826 (Patent Document 4). In this prior-art device, an insulative bonding sheet is inserted between a lead frame and a heat sink, and the lead frame and the heat sink are fixed to each other with the insulative bonding sheet simultaneously with a mold-resin sealing.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-138343 (FIG. 6)
[Patent Document 2] Japanese Patent Application Laid-Open No. 2001-156253 (FIGS. 1 and 3)
[Patent Document 3] Japanese Patent Application Laid-Open No. 2002-76204 (FIGS. 1 and 2)
[Patent Document 4] Japanese Patent Application Laid-Open No. 10-125826 (FIG. 2)
[Patent Document 5] Japanese Patent Application Laid-Open No. 2002-184907 (FIGS. 1 and 2)
[Patent Document 6] Japanese Patent Application Laid-Open No. 2002-128993
[Patent Document 7] Japanese Patent Application Laid-Open No. 2002-53642
[Patent Document 8] Japanese Patent Application Laid-Open No. 10-261744 (FIG. 1)
[Patent Document 9] Japanese Patent Application Laid-Open No. 2000-58575 (FIG. 1)
<Problems of the First Prior Art>
As a material of a mold resin, for example, an epoxy resin with an inorganic filler mixed therein is used. As the percentage of content of the inorganic filler increases, the mold resin has higher thermal conductivity and can more efficiently radiate the heat of the power semiconductor chip to a back side of a module. If the percentage of content of the inorganic filler increases for high thermal radiation, however, the viscosity of the resin contrarily increases, and when the volume percentage of content of the inorganic filler is over 80%, for example, it disadvantageously becomes hard to form the mold resin. As a material of the inorganic filler, silica is generally used, but if alumina, aluminum nitride, silicon nitride or boron nitride, for example, is used instead, it is possible to increase the thermal conductivity of the mold resin to 5 W/mK or higher. Since such an inorganic filler is expensive, however, mixing the expensive inorganic filler in the mold resin which forms the whole of module at high percentage is extremely uneconomical and not a practical technique.
Therefore, in the structure where the mold resin is rounded towards the lower surface of the frame, like in the exemplary structure of the first prior art, there is a limitation in reducing the thermal resistance. Specifically, though it is possible to increase the thermal conductivity of the mold resin by increasing the percentage of filler content, when the percentage of filler content is too high, the viscosity of the mold resin becomes too high and disadvantageously, the mold resin can not be rounded to the lower surface of the frame.
Further, when alumina, boron nitride, aluminum nitride or the like, each of which has high thermal conductivity, is used instead of silica which is generally used as a material of the inorganic filler, the cost of the inorganic filler becomes higher and moreover the amount of required inorganic filler is enormous since the mold resin forms the whole of module, and therefore using the above material for the inorganic filler is so uneconomical.
Accordingly, in the power semiconductor device using the insulating resin of the first prior art, it is hard to increase current-carrying capacity, in other words, to deal a high heat.
Additionally, when the semiconductor device of the first prior art is screwed on the radiating fin, if a foreign matter such as a resin burr exists between a surface of the radiating fin and a main surface of the semiconductor device, there arises a problem that an insulating layer formed of the mold resin may be broken.
Further, when the semiconductor device of the first prior art is attached to the heat sink, such a careful handling is needed as not to damage the insulating layer with a corner or the like.
Furthermore, in an aerobic atmosphere, if the semiconductor device of the first prior art is kept for a long time at a high temperature such as about 200° C., this causes a problem that the mold resin is deteriorated by oxidation.
<Problems of the Second Prior Art>
In the semiconductor device of the second prior art, a ceramic substrate which forms part of a main surface of a package is exposed, and the ceramic substrate is made of a brittle material and there is a possibility that a high stress should be applied to the ceramic substrate to cause breakage therein, depending on the matching of projections and depressions due to the warp in the semiconductor device and that in the radiating fin to which the semiconductor device is attached. This requires a control of size that is stricter than necessary.
When the semiconductor device does not have the exposed ceramic substrate of the second prior art, it is possible to reduce the warp by controlling the ingredient of the filler or the percentage of its content so that the coefficient of linear expansion of the mold resin can roughly approximate to that of a metal frame. Since there is generally a great difference between the coefficient of linear expansion of the metal frame and that of ceramic, however, for example, the coefficient of linear expansion of Cu is 17×106/K while that of Al2O3 is 5×106/K, there arises a problem of warp. This point will be further discussed.
When the ceramic substrate is used, like in the second prior art, there is a problem of warp caused by the difference in coefficient of linear expansion from the mold resin and the temperature change in using the module.
Specifically, the coefficient of linear expansion of Cu is 17×106/K while that of Al2O3 is 5×106/K, and there is a great difference in coefficients of linear expansion between them. Since heating is inevitable in the semiconductor device for controlling high current to which the present invention is directed, a high thermal stress is caused in a junction interface between Cu and the ceramic material by the difference in coefficient of linear expansion and the temperature change in using them, which further causes a warp or a crack.
Thus, when the ceramic substrate is exposed on the back surface side of the device, there is a problem that a thermal stress is caused by a great difference in coefficient of linear expansion and this, accompanying the temperature change, causes a warp and a crack in the ceramic substrate or/and the mold resin. Therefore, there is considerable inconvenience in handling due to inevitable characteristics of ceramic, i.e., high stiffness and brittleness.
Additionally, the package made of the mold resin is formed by implanting a liquid resin into a cavity formed between a plurality of mold dies and in this process, it is inevitable to cause burrs in interfaces between the mold dies. If an insulating resin which is different from the mold resin is used, there is a possibility that the burrs should fall to be left on surfaces of the mold dies when a work is removed after molding. Therefore, there may arise a problem that the ceramic substrate directly presses the burrs, thereby being broken.
<Problems of the Third Prior Art>
In the third prior art, two kinds of resin layers, i.e., the insulating resin layer and the bonding layer, are needed and the thermal resistance necessarily becomes high. Since the bonding layer, in particular, needs a thickness to smooth out the warp or/and unevenness of members in bonding, there is a serious problem that poor adhesion may occur if the bonding layer does not have a thickness of about 100 μm or more and accordingly the thermal resistance necessarily becomes high.
Moreover, the frame is directly bonded onto the bonding layer in the third prior art, and even when a pressure is applied from the insulating resin layer to part of the frame disposed in the bonding surface, the frame is inclined at a fulcrum of other part thereof which is connected to an exterior electrode, and this causes a problem that it is hard to perform a uniform bonding and poor adhesion may occur.
Further, a complex of the insulating resin layer and the metal plate is achieved by coating the metal plate with the insulating resin layer and applying a pressure to the insulating resin layer for a set period of time while heating, and in this process, there are constrains on the device, such as the necessity of air exhaustion to vacuum for avoiding voids, and therefore a batch processing step using a vacuum press device is needed. Though it is possible to use a process of bonding a plurality of metal plates and a plurality of insulating resin layers at one press with released papers or the like inserted therebetween for reduction of cost, there is a limit in the area of metal plate to be processed at one press and therefore it is hard to reduce the cost.
Furthermore, it is very complicate to further layer the bonding layer on the metal plate with the insulating resin layer which is produced by the above method.
<Problems of the Fourth Prior Art>
In the fourth prior art, it is difficult to use an insulating sheet with high thermal conductivity because of a problem in handling and moreover since it is necessary to bond two interfaces between the insulative bonding sheet and the frame and between the insulative bonding sheet and the heat sink at the same time, voids are liable to enter the bonding layer and this is likely to cause problems in withstand voltage characteristics and thermal radiation.
<Summary of Problems>
As discussed above, the prior-art power semiconductor device has the problem that the insulating layer is liable to break depending on how to handle since the insulating resin layer (first prior art) or the ceramic substrate (second prior art) is naked at the back surface. Additionally, the first prior art has the problem that the mold resin is deteriorated when exposed at high temperature in an aerobic atmosphere. The first prior art further has the problem that it is difficult to form a thin insulating layer of a resin with high thermal conductivity due to constraints in molding of the resin and it is impossible to reduce the thermal resistance. Furthermore, the first prior art has a great problem in handling, that the insulating layer is liable to break when the insulating layer is so placed as to come into contact with projections or/and foreign matters in handling the package, or the like.
The power semiconductor device of the third prior art has the problem that the thermal resistance becomes high since the bonding layer is layered on the metal plate aside from the insulating resin layer. The power semiconductor device of the third prior art further has the problem of poor productivity and high cost in the process of layering the metal plate and the insulating resin layer because of the thick metal plate.
The power semiconductor device of the fourth prior art has the problem that the voids are liable to enter the interfaces in a process of bonding the insulative bonding sheet.